The reservoir for pneumococci (Pn) that infect humans is human respiratory mucosa, especially in the nasopharynx. Following deposition on the mucosa, Pn growth and host defenses often reach a balance allowing asymptomatic carriage of the organism for weeks or months. This balance can be upset by events like a virus infection that facilitates bacterial growth causing overt Pn infection and/or enhancing transmission to others. Roles of various host defense mechanisms in controlling Pn carriage are not clearly defined. The purpose of this study is establishement of Pn carriage in healthy young adults, evaluations of which should facilitate development of control measures. Two Pn serotypes (6 and 23) frequently carried by normal persons, and healthy young adult subjects that are at low risk for invasive pneumococcal disease will be employed. Initial studies wil utilize subjects possessing serum antibody to the serotype used. Four subjects will receive 0.1 ml inocula into each nostril with a total dose of 10[1]CFU. At intervals of 2+ weeks, subsequent groups will receive sequential 10-fold increments in dose up to 10[5]CFU, the aim being to establish a dose that causes colonization of 50% of subjects (HCD[50]) has been established without incident in subjects with pre-existing antibody or the 10[5] inoculum has failed, studies will be begun with the lowest dose in subjects lacking serum antibody to the serotype.